The present invention relates to a multi-optical-path photoswitch having pairs of light emitting devices and light receiving devices, in which one of light receiving device is enabled to receive the light in synchronization with emission of the corresponding one of light from light emitting devices in a detection area so as to detect a light shielding state in the detection area. More particularly, the present invention relates to a multi-optical-path photoswitch which is able to avoid incorrect detection which is caused from, for example, occurrence of a multiple light emission state.
The multi-optical-path photoswitch is a switch incorporating a plurality of optical paths or channels constituted by pairs of light emitting devices of a light emitting unit and light receiving devices of a light receiving unit and arranged to be switched on when any one of the optical paths is shielded by an object. Hereto, the foregoing switch has been known as an "area sensor" which is capable of detecting existence of an object over a wide detection area. The multi-optical-path photoswitch is used to improve safety of an operator of a machine tool, a punching machine, a pressing machine, a controller, a molding machine, an automatic controller, a winding machine, a robot, a casting machine or the like. The foregoing multi-optical-path photoswitch is disposed in a dangerous region for a pressing machine or the like to detect shielding of an optical-path which is caused when a portion of the body of an operator, for example, the finger or the hand of the operator, enters the detection area. Thus, the operation of the machine is immediately interrupted or an alarm is issued to protect the operator.
The multi-optical-path photoswitches are disposed along automatic manufacturing lines in a plant to detect existence of moving articles. Thus, the multi-optical-path photoswitches are employed as sensors in an automatic control system with which starts operating a next step if an article is detected.
As a multi-optical-path photoswitch of the foregoing type, for example, the following "multi-optical-path photoswitch (hereinafter called a conventional example), is known. The conventional example has a schematic structure as shown in FIGS. 12 and 13. FIG. 12 is an overall structural view showing the conventional multi-optical-path photoswitch. FIG. 13 is a circuit diagram of a light emitting unit 102. Referring to FIG. 12, the conventional multi-optical-path photoswitch incorporates a plurality of pairs (eight pairs) of light emitting devices 211 to 218 and light receiving devices 311 to 318 forming pairs. In response to a signal supplied from a light-emitting-device control circuit 124, light-emitting-device driving circuits 1291 to 1298 in a light emitting circuit 123 are operated so as to cause the light emitting devices 211 to 218 to sequentially emit light into a detection area. On the other hand, a group of the light receiving devices 311 to 318 is connected to a light receiving circuit 103 having a plurality of input terminals. Only one light receiving device among the group of the light receiving devices 311 to 318, forming a pair with a light emitting device which is currently emitting light, is enabled to receive light by the light receiving circuit 103 in synchronization with light emitting timing of the light emitting device. That is, the light receiving devices are operated at only light emission timing of the corresponding light emitting devices so that each light receiving device does not respond to a light emitted from a light emitting device of which light emitting timing is different from that of the light receiving device on the corresponding optical-path.
In the foregoing case, to sequentially turn the light emitting devices 211 to 218 on, the light emitting circuit 123 has a structure arranged, for example, as shown in FIG. 13. That is, shift registers 1271 to 1278 are connected to the corresponding light emitting devices 211 to 218. Moreover, the shift registers 1271 to 1278 are serially connected to one another. A clock signal is transmitted from a terminal C of the light-emitting-device control circuit 124. At output timing of the clock signal, the states of outputs of the shift registers 1271 to 1278 are sequentially shifted. The foregoing structure is also adapted to the light receiving circuit 103.
Light receipt signals from the light receiving devices 31 to 318 are amplified by received-signal amplifiers, and then supplied to a comparison circuit through a selection circuit having a structure of a shift register and, the light receipt signals are compared with a predetermined reference level by the comparison circuit. If it is detected that a quantity of light received by any one of the light receiving devices is reduced, it is determined that an object has passaged into an optical-path, which is formed by the corresponding light emitting device and the light receiving device, in the detected area.
When one light emitting device is emitting light, only one light-receipt signal from the light receiving device, which forms a pair with the foregoing light emitting device, is made to be receivable and, the reason for this will be described as follow. That is, light emitted from the light emitting device is not always incidented into the corresponding light receiving device. Light with relatively strong intensity might be sometimes received by an adjacent light receiving devices. Therefore, if a structure is designed so that lightreceipt signals from all of the light receiving devices are equally supplied to one comparison circuit, it might be incorrectly determined that the current state is a light receipt state though the optical-path is actually in a light shielded state because of passage of an object.
If the circuit for operating the light emitting devices 211 to 218 in the light emitting unit 102 is suffered from a breakdown, the light emitting devices 211 to 218 are not sequentially turned on at predetermined timing even in a case where the shift registers 1271 to 1278 are normally operated. Therefore, it arises a problem which two or more light emitting devices emit their lights simultaneously (called as "multiple light emission"in this specification). Even in a case where an passaging object shields the optical-path which exists, with this multiple light emission, light emitted from another light emitting device is not shielded. Thus, there is apprehension that the light might be incidented on the light receiving device corresponding to the light emitting device having the light emitting timing. Thus, the light-shielded states cannot be determined correctly because the light receiving unit 103 receives a signal emitted from the light emitting device. In addition to a breakdown of the control circuit and the operation circuit of the light emitting device, it can be considered that multiple light emission occurs if incorrect data is set to any one of the shift registers 1271 to 1278 because of noise or the like.
To prevent the incorrect detection which occurs owning to generation of multiple light emission, the multi-optical-path photoswitch according to the conventional example employs the following technique: that is, the foregoing conventional example has a structure incorporating an electric-current detection circuit 126 for detecting electric currents which are supplied to the light emitting devices 211 to 218. Moreover, the structure incorporates a state detection means 126b for communicating, to outside, occurrence of an abnormal state in the operations of the light emitting devices 211 to 218. The communication is established when the detected electric current levels detected by the electric-current detection circuit 126 exceed a range of load electric currents required to turn the light emitting devices having the turning-on timing on.
That is, the foregoing conventional example have a structure that load electric currents in the plural light emitting devices 211 to 218 which are sequentially turned on at predetermined timing are detected by the electric-current detection means 126. Then, the state detection means 126b determines whether or not the detected electric current levels satisfy the predetermined range. Thus, if a load electric current exceeding the range of the load electric current required to turn one light emitting device on is detected, that is, if a multiple light emission state in which a plurality of the light emitting device are simultaneously turned on at certain light emission timing is realized, the foregoing state can be detected. Thus, reliability of the operation for detecting the light shielded state can be improved.
However, the conventional multi-optical-path photoswitch is adapted to a method in which the sum of electric currents which flow in the light emitting devices is detected to determine occurrence of multiple light emission if an electric current is not lower than a predetermined value of an electric current flow. Therefore, the circuit must have specifications that electric currents which flow in all of the light emitting devices are substantially the same. Therefore, there arises a problem in that setting of different electric current levels to optical paths for the purpose of adjusting effective angular apertures cannot be performed. That is, the multi-optical-path photoswitch is sometimes enabled to easily obtain required effective angular aperture and easiness with which the optical paths can be adjusted by individually setting, to the optical paths, electric currents which flow in light emitting devices corresponding to the optical paths. However, there arises a problem in that the conventional multi-optical-path photoswitch cannot set electric currents individually to the optical paths and design freedom is limited because electric currents which flow in the light emitting devices corresponding to the optical paths are substantially the same.
As described above, the multiple light emission takes place because of supply of an incorrect signal to the light emitting device, for example, setting of incorrect data which occurs because of breakdown or noises of the light emitting circuit (the control circuit and the operation circuit). In addition to the foregoing cause, multiple light emission takes place if metal foreign material has passaged into the apparatus or if output terminals of the control circuit of the light emitting device owning to a bridge of solder which is generated in a process for mounting elements on a substrate. If the control circuit of the light emitting devices incorporates a usual logic IC, an output signal of the control circuit enables a poor voltage amplitude which is about half of "H" level which is generated in a normal case. Also an electric current which flows in one light emitting device is made to be about half. Therefore, the conventional method with which the sum of electric currents which flow in light emitting devices is detected cannot easily detect the multiple light emission.